A. Field of the Invention
The present invention relates to an impeller shell of a torque converter, and in particular formation of the structure of an outer peripheral portion of the impeller shell as well as a method of forming the same.
B. Description of the Background Art
FIG. 4 is a schematic cross section of a typical torque converter known in the prior art. The torque converter is primarily formed of a front cover 3 and three kinds of vane wheels, i.e., an impeller 4, a turbine 5 and a stator 6, and operates to transmit torque from the front cover 3 to the turbine 5 and thereby outputting the torque to an output shaft (not shown) connected to the turbine 5. The impeller 4 has an impeller shell 4a, a plurality of impeller blades 4b attached to the inner side of the impeller shell 4a, and an impeller hub 4c fixed to an inner peripheral edge of the impeller shell 4a. The impeller shell 4a has an outer peripheral portion which is shown by solid line in FIG. 5.
Referring back to FIG. 4, the turbine 5 has a turbine shell 5a and a plurality of turbine blades 5b attached to the turbine shell 5a. When the impeller 4 rotates together with the front cover 3, a working fluid having a velocity energy circulates through the impeller 4, turbine 5 and stator 6 in accordance with the rotation of the impeller 4 so that the turbine 5 absorbs the velocity energy of the working fluid to rotate the output shaft. In this operation, the working fluid in the radially outer portion of the torque converter flows from the impeller 4 toward the turbine 5.
The performance of the torque converter is affected by energy losses as the working fluid moves in a flow path within the torque converter. Energy losses of the working fluid are significantly affected by the configuration of the flow path within the torque converter, the configuration of the vanes and others.
In the torque converter, the impeller 4 and the turbine 5 are opposed to each other, but a space is defined between the impeller 4 and the turbine 5. Therefore, the working fluid, which flows in the radially outer portion of the torque converter from a region proximate the impeller 4 toward the turbine 5, does not entirely flow toward the inner periphery of the turbine 5. Instead, a portion of the working fluid flows toward a position radially outside the turbine 5 through the above mentioned space along a path generally represented by the arrow W. The flow of fluid along the path W is undesirable and causes energy losses in the working fluid.
For suppressing such energy losses, an ideal shape is desired for the inner surface the outer peripheral portion of the impeller shell 4a that has a sectional form represented by a dotted line in FIG. 5. However, in manufacturing processes, the impeller shell 4a is usually formed by press working metal. Therefore, the impeller shell 4a which is a pressed product has a sectional form represented by solid line in FIG. 5. Consequently, a portion of the working fluid flows toward a position or region radially outside the turbine shell 5a (i.e., a region above the shell 5a in FIG. 5) as indicated by an arrow in FIG. 5, which reduces the torque transmission efficiency of the torque converter.